As is known, .gamma.-linolenic acid is one of the essential fatty acids which must be contained in daily foods of human life in the form of the acid per se or a lipid containing the same. It is therefore highly desirable to establish a way for obtaining .gamma.-linolenic acid to be used for the enrichment of foods. Conventionally, .gamma.-linolenic acid or a lipid containing the same is obtained from seeds of evening primrose (Oenotbera biennis L.) but this method is of very low productivity as an industrial method. Accordingly, attempts have been made eagerly to discover an alternative plant which produces seeds containing an oil much richer in the content of .gamma.-linolenic acid or a lipid containing the same than the seeds of evening primrose [see, for example, Journal of American Oil Chemists Society, volume 60, page 1858 (1983)]. Despite the great efforts with this purpose, no very promising plant has yet been discovered and all of the hitherto proposed plants are very specific with low availability. Therefore, industrial applicability can hardly be found in this direction of obtaining .gamma.-linolenic acid with sufficient productivity from plant seeds not only in respect of growing such a special plant but also in respect of collecting the seeds therefrom in large quantities.
On the contrary, there may be a potential possibility for the industrial production of .gamma.-linolenic acid if a microbiological process can be developed and established by culturing a microorganism containing lipids rich in the content of .gamma.-linolenic acid in the microbial bodies. This is because the culture of microorganisms requires no solar energy and can be performed as a factory process so that the process is free from the restrictions imposed by the availability of a large area of plant site and the meteorological conditions to give a high and stable productivity and enjoys free controllability of the production rate.
It has been discovered that certain filamentous fungi belonging to the genus of Mortierella are very promising in this regard including the species of M. isabellina, M. vinacea, M. ramanniana, M. ramanniana var. angulispora, M. nana and the like. Namely, it has been discovered that the fungus bodies of these filamentous fungi cultured in a culture medium containing carbohydrates as the carbon source contain 30 to 60% of lipids on the dry basis of which the content of .gamma.-linolenic acid is as high as 2 to 12% or, in most cases, 7 to 8% in the overall fatty acid content. Further advantageously, these filamentous fungi can be cultured in an unusually high density in a culture medium so that the productivity of .gamma.-linolenic acid by the microbiological process can be increased very greatly. Conventionally, the lipids contained in the fungus bodies are extracted as a whole by homogenizing the fungus bodies in a solvent mixture of chloroform and methyl alcohol so that the content of the .gamma.-linolenic acid glycerides in the extracted lipids is also limited.
Although the above mentioned microbiological process is very promising as an industrial method for the production of .gamma.-linolenic acid, the content of .gamma.-linolenic acid in the fungus body lipids is still not high enough to ensure the usefulness of the fungus-origin lipids extracted in a conventional manner as such as a material for food enrichment or other medicinal purposes. When the culturing conditions of the filamentous fungus are modified with an object to increase the growth rate of the fungus bodies or to obtain the fungus bodies containing an increased overall amount of lipids, in particular, it is sometimes difficult to have the content of .gamma.-linolenic acid in excess of 3 to 6% in the overall fatty acids. Nevertheless, no efficient method has yet been developed for the selective extraction of the lipids from the fungus bodies into fractions rich and lean in the content of .gamma.-linolenic acid or no convenient method for upgrading the lipids extracted from the fungus bodies is known in respect of the content of .gamma.-linolenic acid.
Accordingly, it would be very desirable to develop a method for obtaining lipids containing an increased amount of .gamma.-linolenic acid from the bodies of cultured filamentous fungi. Such a method would be useful also for the enrichment of the lipids obtained from the seeds of plants, e.g. evening primrose, in respect of the content of .gamma.-linolenic acid or glycerides of the same.